1. INDOT Culvert Sizing Policy
Purdue Road School
March 9, 2011

2. Your Presenters: John Bartolo, PE & Crystal Weaver, PE
INDOT Office of Hydraulics

3. The Goal for Policy is a Clear Understanding!

4. Design Storm Frequency
Existing roadway serviceability should be maintained. For example: if roadway serviceability for a lightly traveled two-lane facility is already above Q100, then the Q10 or Q25 should not be used.

5. Allowable Headwater New Alignment
0.14 ft. Maximum Backwater
Special Cases:
Backwater Dissipates 0.14 ft. or Less at R/W
Backwater is Contained in the Channel
Remember to Check Outlet Velocity!
Flood easements are purchased
Very rare and only on large projects
0.14 ft maximum backwater is required by DNR, if a DNR Construction in a Floodway permit is required.
0.14 ft maximum backwater is also required per INDOT policy, if a DNR Construction in a Floodway permit is not required.
DNR will accept backwater dissipating to 0.14 ft at the R/W and flood easements, but will probably not accept backwater above 0.14 ft, even if it remains within the channel.

6. Allowable Headwater Replacing an Existing Culvert
Match or Reduce Existing Backwater
Exception:
Existing Backwater Exceeds 1 ft.
If existing backwater exceeds 1 ft, the maximum proposed backwater is 1 ft.

7. Allowable Headwater Other Constraints
Grade of Adjacent Driveways
Finished Floor Elevation of Adjacent Buildings
Elevation of Surrounding Crop Land
Grade of adjacent driveways – don’t want water flowing backwards down the driveway.
Finished Floor Elevation of adjacent buildings – if less than 2’ higher than Q100, can trigger the need for a DNR permit. Also may limit the amount of backwater we can accept for a proposed structure.
Elevation of surrounding crop land – don’t want water sitting on crops for more than 24 hrs – will cause damage.

8. Allowable Headwater Other Constraints

9. Roadway Serviceability
For a Q100 Design Storm
2 ft. Below Edge of Pavement
For Design Storms Less Than Q100
Headwater Can Not Exceed Edge of Pavement

10. Maximum Outlet Velocity
Revetment Riprap < 6.5 ft/s
Class 1 Riprap > 6.5 ft/s < 10 ft/s
Class 2 Riprap > 10 ft/s < 13 ft/s
Energy Dissipator > 13 ft/s
See Chapter 34 in the Indiana Design Manual
Prefer to keep maximum velocity below 13 ft/s

11. Minimum Outlet Velocity
Typical Minimum Outlet Velocity
3 ft/s
Minimum outlet velocity helps prevent silting.

12. Do Not Consider Storage at the Culvert
Storage at the culvert should not be considered because it is generally outside of INDOT’s control. Since there is no guarantee that the storage facility will be present through the entire life of the structure, the culvert should be sized as if it were not present. This is a conservative solution.

13. Priority System Trial 1 - Single Circular Pipe
Trial 2 - Single Deformed Pipe
Trial 3 - Single Specialty Structure
Trial 4 - Multiple Circular Pipes
Trial 5 - Multiple Deformed Pipes
Trial 6 - Multiple Specialty Structures
Generally speaking, the designer won’t go back down a step in the Priority System. i.e. If the Structure is already a box culvert (Trial 3), a Single Circular Pipe (Trial 1) is probably not going to be a feasible option.
Multiple pipes should be avoided, if at all possible. They present a maintenance problem, due to the twinned structures catching debris in the middle. Also, they can catch debris at high flow and lead to culvert failure.

14. Pipe Culvert Interior Designation
Smooth Interior:
Manning’s n = 0.012
Corrugated Interior:
Manning’s n = 0.024
Smooth interior - Manning’s n of should also be used for CIPP and HDPE liners.

15. Minimum Culvert Size Structure Application Minimum Circular Pipe Size
Deformed Pipe Size
Driveway Culvert
15 inch
1.1 ft2
Mainline/Public Road
Approach Culvert
(2 lanes)
(3 + Lanes)
36 inch
6.7 ft2

16. Culvert Extension Process
Match Existing Culvert Size and Type
Perform Appropriate Hydraulic Analysis

17. Culvert Sumping Three Sided Structures
Sump structure
18 inches for sand
12 inches for other soils
3 inches for rock or till
Structure should not increase in size
See standard drawing for stream bed and scour information

18. Culvert Sumping Boxes and Pipes

19. Culvert Sumping Boxes and Pipes
Structure may increase in size, if sump is more than 3 inches
Hydraulic modeling may be needed to check capacity.

20. Approved Design Methods
Computer Method
FHWA Program HY-8
Hand Method
Nomographs From FHWA Publication HDS #5 “Hydraulic Design of Highway Culverts”
Available as a Printed Manual, on CD ROM or online

21. Design Responsibility
All Types of Projects
Consultant Will Size the Culvert(s)
INDOT Hydraulics Will Review Computations

22. Where Do I Get More Information?
INDOT Design Manual Part IV Hydrology and Hydraulics
FHWA Hydraulics Home Page
Hydraulics Engineering Publication List
Hydraulics Engineering Software Reference List

23. Questions